It is well known to cultivate plants in a natural or artificial growth substrate., in particular a mineral wool growth substrate, such as rock wool or glass wool. Water and, if necessary, fertiliser are supplied to the mineral wool substrate, generally by causing water, optionally containing fertiliser, to flow through the substrate.
In systems of this type it is important that the roots of the plants receive an appropriate supply of oxygen. The level of dissolved oxygen in the water around the roots of the plant (water oxygen level) is important because this determines the oxygen available to the plant.
An oxygen sensing device is known in the food industry. Such a device has been described in “A Novel optical Method to Determine Oxygen in Beer Bottles”, Draaijer et al, European Brewery Convention 27th Congress 1999, for determination of oxygen in food containers such as beer bottles. This method is also discussed in “An optical oxygen sensor”, Draaijer et al, 2nd Inter-Regional Conference on Environment-Water, 1 to 3 Sep. 1999. In this latter paper use for monitoring oxygen levels in groundwater and mineral wool growth substrates is suggested. For the latter use a fibre optic version is recommended. WO 01/63264, published 30 Aug. 2001, discloses an optical sensor for measuring oxygen levels, primarily for measurement of such levels in consumable products stored in glass.
However, the requirements for water oxygen levels are complex and simply determining water oxygen level is not generally sufficient to tell the grower whether or not conditions are optimum. For instance, a given oxygen content might be acceptable for one type of plant but not another, or might be acceptable for a plant at one stage of its growth and not another, or low water oxygen level (ie. low water oxygen content) in a system having a high water replacement rate may not be problematic whereas water oxygen level would be problematic in a system having lower water replacement rate. Thus there is a complex interaction between the various growth conditions which determines whether or not a given water oxygen level is acceptable.
In the past, growers have not monitored plant growth conditions in a truly systematic manner. In general, a visual observation of plant conditions and, in some cases; measurement of water content of the substrates in which the plants are grown is used to assess whether conditions require modification. If this rather imprecise observation method suggests that conditions are not optimal, changes can be made for instance in the content of water in the substrate or the rate of water supply. In the past, although growers have been generally aware that water oxygen level is a potential factor in the suitability of growth conditions, they have not focussed specifically on water oxygen level and its measurement as a means of assessing whether growth conditions are optimal.